1. Field of the Invention
The present invention relates to elements for use in fiber optic-based systems of the type that are limited to lowest order optical mode transmission. More particularly, this invention pertains to improvements in such elements that result in operation over a broad wavelength range.
2. Description of the Prior Art
Numerous optical systems rely exclusively upon the transmission of single or lowest order mode light. For example, long distance fiber optic communication systems utilize single mode fiber to economize by increasing the distances between, and, thus, minimizing the number of, repeaters. Single mode fiber can maintain the character of transmitted signals over greater distances by offering larger bandwidths than multimode fiber.
In the past, optical signals of single mode systems have been processed by integrated optical devices, such as the Mach-Zehnder amplitude modulator of FIG. 1, that employ an exclusively single-mode design criterion in which all waveguides are capable of supporting only the fundamental mode. This has been believed necessary to avoid problems related to extinction (on/off) ratio, spatial uncertainty and the like. If one were to attempt to design an amplitude modulator of the Mach-Zehnder variety for operation from 1300 nm to 1550 nm, a very important wavelength band, using a conventional device optimized for 1300 nm operation, the weaker waveguide confinement at the longer wavelength would give rise to increased bend losses as well as require a higher drive voltage. Alternatively, using a 1550 nm optimized device at 1300 nm, one would expect to find a very poor extinction ratio as a consequence of multiple modes at the shorter wavelength.
Analogous problems are associated with other optical waveguide devices. As a result, one is presently forced to employ circuit elements optimized for very narrow wavelength range of operation, limiting interchangeability between wavelengths of operation.